This invention relates to the continuous heat processing of free flowing bulk materials in a vertical shaft furnace. Feed materials usually having a greatest dimension smaller than one inch are introduced to the top of a furnace to form a bed of such materials, or burden. The burden moves downwardly under the influence of gravity and the treated particulate product is removed from the bottom of the furnace thereby setting up a gravitational flow of material. The rate of material throughput is determined by the rate of discharge of the product.
Heated process gases are introduced around the periphery of the shaft furnace and flow upwardly through a treating zone in counterflow heat exchange with the descending burden. Thus the product leaving the treating zone has a temperature essentially equal to that of the incoming heated process gas.
In the continuous direct reduction of iron oxide to iron in a vertical shaft furnace it is necessary to cool the product prior to its discharge. It is thus desirable to improve the economics and thermal efficiency of such a process by recuperating a portion of the heat content of the product.
I have developed an improved shaft furnace apparatus having means for injecting a cold gas stream into a hot packed bed in a zone beneath the heat processing zone. The cold gas stream takes on heat from the heated burden, the burden becoming cooled thereby. The stream of cold gas is introduced around the periphery of the furnace but below the hot process gas inlets. The cold gas stream follows a path through the burden roughly parallel to that of the hot process gas stream with virtually no intermixing. As the gas reaches the center of the burden it moves upwardly, and eventually exits the burden at the stock line from the central region of the top of the furnace.
As the cold gases enter the furnace, they contact hot particles, which have been previously heated by the main hot process gases. These particles have sufficient area and heat content to raise the temperature of the cold gases to essentially that of the burden before these gases enter the processing zone.